Bulk Reef Supply

BRS Bulk GFO Granular Ferric Oxide

Grouped product items
1/4 Gallon (1 lb) BRS Bulk GFO

SKU: 000600

In stock


- +
1/2 Gallon (2 lbs.) Bulk GFO

SKU: 000601

In stock


- +
1 Gallon (4 lbs.) Bulk GFO

SKU: 000602

In stock


- +
5 Gallon (20 lbs.) Bulk GFO

SKU: 000603

In stock


- +

Why does BRS recommend this?

Phosphates are going to be one of the biggest battles we have with a reef aquarium.  The BRS GFO works great in filter socks and reactors.  For the price Standard Granular GFO is going to be one of the best values when it comes to GFO for phosphate removal.


Granular Ferric Oxide (GFO) removes phosphate from the aquarium and is one of the most commonly used filtration media used in the aquarium industry.  GFO is one of the few ways to easily maintain ultra low phosphate on a consistent basis. Maintaining these ultra-low levels will help prevent algae outbreaks as well as treat existing algae issues. Your aquariums glass will also stay clear and free of the green hue significantly longer. GFO is most commonly used in a media reactor like the BRS reactor or media bag.


Phosphate inhibits proper coral growth by incorporating itself into the corals skeletal structure which makes it difficult for the coral to grow by laying down additional calcium and carbonate (alkalinity) ions. Maintaining ultra low phosphate levels will increase the growth of any SPS or LPS coral.


Algae outbreaks are one of the most common reasons for a complete tank shut down. We recommend preventing them by maintaining an ultra-low nutrient level environment where it would be difficult for them to get out of control to begin with. It is much easier to prevent outbreaks than it is to treat existing outbreaks.


GFO comes in two types:


  • Granular GFO is varied in its shapes and requires the least amount of flow to tumble. Good for reactors.
  • High Capacity GFO is twice as dense as Granular GFO, so twice as much material will fit into a reactor. It's extremely hard and has less dust to begin with. Fewer fines will be created during use and transportation. By volume High Capacity GFO will remove roughly twice the phosphate compared to Granular GFO. Best overall performance.


Special note on fighting existing algae problems: Algae needs three main nutrients to grow: phosphate, nitrate and light. Reducing any one of these will significantly slow down algae growth but may not completely solve your issue. Once algae takes hold, it can be a difficult battle but it is winnable. The best offense against algae is to take preventative measures and attack before an outbreak is apparent. Use the following suggestions and be aggressive if an algae problem is already present:


  • Maintain undetectable phosphate levels with good feeding habits and use of a phosphate remover like GFO.  99% of all phosphate is added via foods added to the tank.
  • Control nitrate levels by reducing feedings, increasing the water change schedule and maintaining a properly sized protein skimmer.
  • Use nutrient-free RO/DI water for water changes and top off water
  • Shorten your lighting period or intensity. In some cases aquarists have found replacing old bulbs that have fallen out of their intended spectrum helps as well
  • Continuously remove as much algae as possible by hand.
  • Add predators nothing helps an algae outbreak as much as critters who eat it all day long. Various tangs, lawn mower blennies, crabs and snails are all good options. It is also theorized that a healthy pod population will also control algae growth before it gets a chance to take root.

Note: All Bulk GFO is packaged by weight


GFO Instructions

Write Your Own Review
You're reviewing:BRS Bulk GFO Granular Ferric Oxide
Your Rating
Only registered users can write reviews. Please or

Product Questions

I just bought the high capacity gfo. On the instructions you have on the brs website it says to start with 1 tbsp per 8 gallons but on the actual bottle I received it says to start with 1 tbsp per 4 gallons. Did I just put too much in? Why is there a discrepancy like this?
Question by: Jared Engberg on Oct 20, 2023, 2:17 AM
Hey there,
Thank you for reaching out to us. The new instructions on the package are correct.

Please let me know if you need anything further and take care!
Answer by: Matt Lucero (BRS Staff) on Nov 16, 2023, 7:13 PM
I am planning to use GFO in a canister filter (because I already have the filter lying around). Which of your GFO products would be better for this type of application?
Question by: Kim on Apr 7, 2020, 2:46 PM
Hi Kim,

While running GFO in a reactor is more efficient, either the standard or High Capacity GFO will work in a canister filter provided you place it in a mesh bag. Try to position the bag in such a way that water flows through the media rather than around it for best results.

I hope this helps!
Happy Reefing
Answer by: Nick on Apr 13, 2020, 12:47 PM
How much GFO would I need to use in a BRS reactor? I have a 120G display tank with a 40G Sump.
Question by: Logan Kreider on Nov 21, 2017, 9:41 AM
First you will need to estimate your total system water volume. That number will be your display tank minus any rock displacement and also your sumps water volume too. In most cases just using the total volume of your display tank will be a pretty close guess. For a 120 gallon system you will want to use 1.88 cups of GFO. If you think you have a different volume, you can always use our Reef Calculator (link below) and input your specific values. : )

Answer by: Charlie on Nov 22, 2017, 2:21 PM
Will this work with a 200 micron filter sock? or will too much be released into the water column? what if GFO gets released into the water? What should be done?
Question by: Joey on Oct 23, 2017, 10:58 AM
Hey there, thanks for reaching out to us!

Provided the flow into the sock isn't so strong that it tumbles the media excessively you should be OK. If the media is tumbled to the point where it begins to break up/you notice brown dust on the filter sock, I would use a reactor for the media or place it in a mesh bag in your sump or back chamber of your tank.

The fine particles should not have any negative effects on your fish and coral provided it is only a small amount. There have been anecdotal accounts of a large amount of fine particles having a negative impact but these seem to be limited to extreme cases.

Removal of the fine particles can be achieved with filter socks, filter floss, or any other fine mechanical filtration.
Answer by: Nick on Nov 10, 2017, 10:30 AM
What is the flow requirements of the standard GFO compared to Phosguard?
Question by: Tyler on Jan 24, 2017, 7:06 PM
Great question! You will need enough flow to tumble the GFO inside of a reactor. That flow rate will be determined by how much GFO media you need for your specific tank. You can use our GFO calculator to find your tank's requirements. :-)

Answer by: Randy on Jan 28, 2017, 7:47 AM
Can I use BRS Bulk GFO Granular Ferric Oxide for freshwater
Question by: Andrii Bat on Aug 17, 2016, 5:49 PM
Hey Andrii,
You can use gfo in both salt and freshwater, it will remove p04 the same.
Answer by: Jason Smith on Aug 18, 2016, 2:55 PM
How many gph should I run through the GFO for a 40 gallon tank
Question by: Arvard Fowler on Jul 16, 2016, 10:50 PM
Hi there,
The GPH through the reactor will need to be set for the volume of GFO so that it has a slight simmer at the top of the GFO in the reactor. If the GFO if not moving it may turn into a brick pretty quickly, or if the tumble is too vigorous then it will end up as as dusty fines from tumbling against itself too hard.
Answer by: Charlie on Jul 18, 2016, 10:27 AM
On 6/22 my phosphorus reading was 3 ppb. On 6/23 reading was 8 ppb, so I started running my BRS GFO reactor. On 6/25 reading was 12 ppb.Today it's 15. Should I change out the gfo? I removed 60 lbs. of sand on 6/20 to reduce nitrate & algae. Thanks
Question by: Terry Dangler on Jun 28, 2016, 1:10 PM
Great question!
Depending o the changes occurring in the system you may need to change the GFO if the phosphate continues to rise. In systems with more phosphate the GFO will certainly deplete much faster and will need to be changed. Major tank changes can stir up a lot of left over waste which may contribute to the excess levels.
Answer by: Connor on Jul 1, 2016, 9:53 AM
Is there any aluminum in this? My new toadstool hasn't opened In 3 weeks.
Question by: Patti on Jun 11, 2016, 9:08 PM
Great question!
This does not contain aluminum and from our use throughout the years unless overdosed to shock the system GFO does not tend to have a negative impact. Feel free to let us know if you have any further questions!
Answer by: Connor on Jun 14, 2016, 5:53 PM
Hi, can you place the standard GFO in a media bag then place into a media reactor to reduce the fines released into the water.
Question by: Marc Upton on Jun 1, 2016, 12:59 PM
Certainly can however for best performance the GFO should be rinsed before hand and have a sponge on top so it can tumble. Feel free to let us know if you have any further questions!
Answer by: Connor on Jun 2, 2016, 12:18 PM
Does your BRS GFO have a shelf life?

Question by: jose palomino on May 23, 2016, 8:07 AM
Great question! The GFO has no shelf life, so long as you keep it dry, it should last indefinitely.
Answer by: Carter on May 25, 2016, 2:59 PM
I run a 90g tank, I just purchased the dual reactor, and was curious why I can't just put 2 cups in the reactor instead of 1.5 cups.
Question by: David Barr on Apr 27, 2016, 8:31 PM
Great question!
The recommended amount of GFO is 1.41 in accordance with our instructions and the reef calculator. We recommend running the correct dosage when ever possible as it will limit the impact as well as be much easier to tune and tumble.
Answer by: Connor on Apr 28, 2016, 10:50 AM
I am trying to understand how much flow should go through GFO. Should there be enough flow to where the whole column of GFO is slightly tumbling or just enough to where only the surface is tumbling?
Question by: Evan Chandlee on Mar 21, 2016, 5:46 PM
Great question!
There should ideally be enough flow to keep the entire column tumbling to prevent clogging. If clogging occurs try removing some media and adjusting the flow. Feel free to let us know if you have any further questions!
Answer by: Connor on Mar 22, 2016, 11:16 AM
If my tank is mainly sps dominated would this gfo work well? Or high capacity?
Question by: Sergio on Jan 17, 2016, 10:45 PM
Great question!
Both will work well on your system! However the High Capacity is more efficient while using less space. The regular will take up more space. Feel free to let us know if you have any further questions!
Answer by: Connor on Jan 26, 2016, 6:56 PM
Is GFO the same thing as phosban ? Can it be used in two fishes reactor ? Should it tumble in the reactor? thanks
Question by: GH on Aug 4, 2014, 3:51 PM
GFO is Granular Ferric Oxide, it is the name of the actual material. Phosban happens to be just a brand name of a GFO product (among others). It can certainly be used in a phosban reactor and you do want to tumble it or it will clump together into a big rock.
Answer by: Bulk Reef Supply on Aug 4, 2014, 3:51 PM
How do you know when it is time to replace the GFO in your reactor?
Question by: Ken on Jul 16, 2014, 3:01 PM
Hi Ken,
The best way to know would be to test your water for phosphates using an easy to read phosphate meter like the hanna checker. When phosphates start to increase it would be a sign that the media is depleted and should be changed.
Answer by: Bulk Reef Supply on Jul 16, 2014, 3:01 PM
Does BRS GFO Granular lower alkalinity?
Question by: Jason on May 13, 2014, 4:15 PM
Hi Jason,
It wouldn't be likely to see any negative effect on your calcium or alkalinity. Its worth noting though, phosphate prevents precipitation (for example the formation of coral skeletons), so by removing phosphates your corals can grow easier which would mean they consume more calcium and alkalinity.
Answer by: Bulk Reef Supply on May 13, 2014, 4:15 PM
How long should the GFO remain in the reactor? The fake stuff at the stores say 7 hours. Thank you for your time.
Question by: Jason on May 11, 2014, 7:59 AM
Hey Jason,
It depends on how much phosphates you have. The more you have, the faster it will need to be replaced and vice versa. As a whole though 4-6 weeks would be pretty typical. The best thing to do would be to monitor your phosphate levels with something like a Hanna phosphate checker. When the phosphates begin to rise that would indicate your GFO is used up and its time to change it.
Answer by: Bulk Reef Supply on May 11, 2014, 7:59 AM
I have a biocube 29 and was wondering if this product can be used with some sort of a bag hanging in one of the back chambers of my tank? Will that be effective. I do not have the space or the recources to get a reactor.


Question by: John on Apr 6, 2014, 11:05 AM
Hi John,
It won't be as effective as a reactor and you will want to periodically knead the bag, but it would certainly be better then nothing.
Answer by: Bulk Reef Supply on Apr 6, 2014, 11:05 AM
starting a 90 gallon . will also buy your reactor , how much product do I need to start out to fill the reactor. Thank John
Question by: John Lasky on Mar 1, 2014, 11:28 AM
Hi John,
The amount of media depends on the size of your tank with the maximum capacity of the reactor being 2 cups of media, though fortunately with your size tank you wouldn't be in excess of that. For a 90g tank you would use 1.5 cups of media. The easiest way to get measurements for most of these products is to use the BRS Calculator which can be found right here:

Answer by: Bulk Reef Supply on Mar 1, 2014, 11:28 AM
How many cups in a pound of the BRS GFO?
Question by: Jeff on Jan 2, 2014, 4:49 PM
Hi Jeff,
There is 4 cups in our 1lb container, 8 cups in the half gallon/2lb container and 16 cups in a gallon/4lb container.
Answer by: Bulk Reef Supply on Jan 2, 2014, 4:49 PM
Sorry I didn't see the instructions, can you tell me how much I will neeed to use in your BRS reactor and how long between changeouts? I have a 90 gal tank and soon to be 125
Question by: Paul on Nov 27, 2013, 10:19 AM
Hey Paul,
The easiest way to know is to use the reef calculator at the link below. You just enter the size of your tank and it will tell you how much to run. In the case of your 90g thats about 1.4 cups (22 tablespoons). How long it lasts depends on how much phosphates you add to the tank (which is largely a matter of how much you feed). Pretty typical is 4-6 weeks though.
Answer by: Bulk Reef Supply on Nov 27, 2013, 10:19 AM

I want to mix GFO with carbon in the BRS single reactor. Is it OK to use the cheaper GFO to mix in with the carbon or do you need the high capacity GFO? I have a small system so I use low quantities of both carbon and GFO in my reactor. I was just worried about the regular GFO being too small.


Question by: James on Nov 19, 2013, 7:22 AM
Hey James,
It doesn't cause any issues with the reactor to use standard GFO. Really the choice just comes down to the volume of space you have to work with. If you have a large tank you may need more regular GFO then will physically fit in the reactor. Then you either need to get a larger reactor, or use the HC GFO which only requires half the amount.
Answer by: Bulk Reef Supply on Nov 19, 2013, 7:22 AM
Should GFO be changed just as often as Carbon?
Question by: michael on Sep 9, 2013, 1:58 AM
Hey Michael,
GFO should be changed when it is depleted. How long this takes will depend on the amount of phosphate that is added to the tank. A pretty typical tank that would usually be 4-6 weeks or so but the best way to tell is to monitor the phosphates. When they start to increase that would indicate that DI resin has reached its capacity.
Answer by: Bulk Reef Supply on Sep 9, 2013, 1:58 AM